The overall goal of this project is to develop dental restorative resin composites that retain the desirable properties of current materials while having vastly improved polymerization shrinkage and while generating much lower interfacial stresses during curing to a bonded tooth surface. The expected benefit will be the availability of a resin composite that, for the first time, will be a true amalgam replacement suitable for use in even large posterior restorations. Key to the work is the solution of problems elucidated in previous studies including the tendency for LC monomers to crystallize when highly filled and the increase cure shrinkage of LC monomers as the cure temperature approaches the upper temperature limit of the nematic mesophase stability range. The specific aims include the synthesis and evaluation of new monomers (1) which, when blended, will not crystallize over protracted periods, (2) which when blended will yield nematic stability ranges extending below room temperature and above 50 @C, and (3) which contain an inserted flexible region for improved toughness of the cured resins. The first and second aims will be achieved by synthesizing monomers of the form A-s-ph-COO-phs-COO-s-phs-COO-ph-COO-s-A where A is a polymerizable acrylate or methacrylate group, s is a spacer group (e.g., COO(CH2)6 COO) and ph is a phenyl group and phs is a substituted phenyl group (e.g., t-butyl-ph). These monomers will be blended with previously synthesized monomers in designed partial factorial experiment and the cure shrinkage, stress on cure, and mechanical properties determined. The third specific aim will be addressed by synthesizing monomers of the form A-s-ph-COO-phs-COO-ph-O-[Si(Me2]n-ph-COO-phs-COO-ph-s-A. Again, these monomers will be characterized alone and blended with other monomers for polymerization shrinkage, cure stress, modulus of elasticity, transverse strength, and fracture toughness. Selected monomer blends will be formulated into filled systems containing ca. 80 wt. Percent coupled inorganic filler and the cured properties determined (as above), plus wear resistance and shelf life. The best monomers will be formulated into optimized filled systems with three fillers: fused silica, heterogeneous microfiller and small particle radiopaque glass in Core 2.